1. Field of the Invention
The present invention relates to an ophthalmological measurement apparatus for use in a medical institution, such as an ophthalmological clinic, which examines the fundus of a subject's eyes.
2. Description of the Related Art
(1) A laser doppler flowmeter is a known ophthalmological measurement apparatus for measuring the velocity of a bloodstream in a blood vessel in the fundus of a subject's eyes. The fundus blood flowmeter directs a laser on an arbitrary blood vessel in the fundus of the subject's eyes, and detects a doppler shift of the laser light reflected from the bloodstream within the blood vessel. During measurement, the flowmeter keeps track of the same blood vessel to direct the laser light to the same blood vessel even when the eyeballs move. Photodetectors receive an interference signal between the doppler shift component of the reflected light from the bloodstream and the light from the sill vessel wall and the signal is frequency analyzed to determine a doppler shift frequency. Let .DELTA.fmax1 and .DELTA.fmax2 represent maximum doppler shift frequencies of the received signals from the two photodetectors, let represent the wavelength of the laser, let n represent a refractive index of a measurement point, let .alpha. represent an angle made between two receiving optical axes within the eye, and let .beta. represent an angle between a plane of the two receiving optical axes within the eye and the velocity vector of the bloodstream. The blood stream velocity (maximum velocity Vmax) is quantitatively determined by the following equation. EQU Vmax={.lambda./ (n.multidot..alpha.)}.multidot..vertline..vertline..DELTA.fmax1.vertline.- .vertline..DELTA.fmax2.vertline..vertline./cos .beta. (1)
By making measurements from two directions in this way, the directional factors of the measurement lights cancel each other out. The bloodstream in an arbitrary point in the fundus is thus measured. By making the direction of the velocity vector coincide with the line where the fundus intersects the plane in which the two receiving optical axes lie, thus to make .beta.=0.degree., namely cos .beta.=1, a true maximum blood stream velocity is measured.
(2) The maximum doppler shift .DELTA.fmax is expressed as .vertline..DELTA.fmax.vertline. with its sign information dropped. When the bloodstream velocity is measured at different blood vessels in the fundus, the maximum frequency shifts .DELTA.fmax1, .DELTA.fmax2 may take three combinations of signs: both positive, both negative, and one positive and the other negative. As understood from Equation (1), determining the maximum bloodstream velocity Vmax becomes impossible, depending on the region of measurement. To resolve this problem, two points of incidence of light are set up at two direction to a spot image on the pupil, and maximum frequency shifts .vertline..DELTA.fmax1.vertline., .vertline..DELTA.fmax2.vertline., .vertline..DELTA.fmax1'.vertline., and .vertline..DELTA.fmax2'.vertline. are determined from the optical paths from the spot image, and the maximum bloodstream velocities Vmax and Vmax' are thus determined. By comparing the two maximum bloodstream velocities Vmax and Vmax', a proper angle of incidence of a light beam to determine a true maximum velocity is determined. Based on this information, the optical paths are selectively switched and an actual measurement is performed.
From the clinical standpoint, it is quite useful to observe changes in the subject's eyes with time. To fix the same region on the same blood vessel at the next time, a fundus image is recorded in a video cassette recorder or a video printer during a fundus bloodstream measurement, and the same region is visually determined referring to the measurement point position in the fundus image next time.
(a) In the conventional technique (1), however, the bloodstream velocity changes periodically in an artery in synchronization with the heart beat cycle of contraction and expansion. A measurement of at least one period is required on an artery while a measurement of short time is sufficient on a vein because of no substantial velocity change therewithin. Conventional measurements are performed without paying attention to the difference between the artery and the vein. A long measurement time, set for artery measurements, is too long for vein measurements. A subject is advised against blinking throughout the time. The fundus of the subject's eye are thus subject to a higher dose of light exposure than actually required.
Fast Fourier Transform (FFT) is typically used in the frequency analysis of doppler shift signals. To enhance the resolution of the frequency, a large quantity of time-series data is needed. The artery changes periodically, causing the bloodstream velocity to sharply rise at each contraction phase of the heart. If an excessively large quantity of data is used in the frequency analysis, velocity variations of interest will be obscured. On the other hand, the vein is practically subject to no such periodic velocity variations, and the use of a great deal of time-series data results in an increase in the frequency resolution in frequency analysis. In frequency analysis, consideration has to be given to acquiring data over a duration of time during which the velocity variations of interest in the artery are not obscured.
(b) In the conventional technique (2), a received signal may be affected by noise when the subject's eyes suffer from cataracts or when an eyelash is in the way during measurement. As a result, an accurate measurement will not be obtained in the determination of .DELTA.fmax1 and .DELTA.fmax2. From a cursory check, one cannot tell whether the measurements contain unwanted noise components possibly arising from cataracts or eyelashes, and thus one cannot determine whether the measurements are a true fundus bloodstream velocity. Since test personnel possibly observe the cataracts or eyelashes in the subject's eyes during a measurement, they may record that fact as reference information. The test personnel, however, are likely to forget recording in the course of busy measurement work.
Any particular manual work intervention, if introduced in the determination of the region of measurement, will increase error factors and decrease repeatability in measurement. The measurement work itself will be complicated and waste a lot of time. The region of measurement may be determined by observing and recording the fundus image and measurement spot using a CCD camera and by measuring the images. Such a measurement requires a complex and costly system.